For many applications in the fields of biology, medicine, and pharmacology, very thin sections of tissue from living animals are necessary, e.g., for microscopic examinations. To cut such sections, specialized precision cutting apparatuses called microtomes are employed.
Specimens from which tissue sections are to be cut are prepared by various techniques, e.g., by embedding the specimen in paraffin wax, plastic, or other suitable solid matrix which has the proper degree of hardness for sectioning with a glass knife. A further known method for specimen preparation is the so-called cryostat method in which the specimen is frozen and sectioned in the frozen state.
In preparing biological specimens for electron microscopes, it is essential that very thin sections of tissue be cut in order to properly examine the specimen. It is often desirable that ultra-thin sections of a thickness on the order of 200 to 300 Angstrom units or less be cut from biological tissue. Ultra-thin sections must be cut without distorting the specimen through compression of the tissue or scoring of the tissue by the cutting tool edge.
In preparing specimens for an electron microscope, a microtome having a knife formed from plate or window glass (or diamond) is utilized to section the specimen into very thin pieces. To form the knife, the plate or window glass is fractured to expose a sharp edge.
After the biological specimen has been prepared for sectioning, the embedded specimen is secured to the specimen holder of a microtome. The microtome causes the knife edge to contact the specimen so that very thin sections of the specimen are sliced therefrom.
Advances in microtomes involve the mechanical structure which produces the relative movement between the specimen and knife to section the specimen, and the manner in which the specimen is supported.
U.S. Pat. No. 664,118 discloses a microtome wherein a horizontally movable slicing knife (B) is clamped to a supporting bar (C). The bar is pivotally connected to horizontally-swinging arm (D and D'). Movement is actuated by a horizontally-swinging hand lever (F). The specimen is frozen on the supporting block (G) and the support can be raised or lowered toward the knife by a screw.
U.S. Pat. No. 2,292,973 discloses a microtome mounted on supports such that when handle (19) is moved back and forth, swinging arms (14 and 15) for the knife and swinging arms (12 and 13) from the support, join together to form a parallelogram so that the object to be sliced always remains in the same position. The sections are also uniform in thickness as the set screw is automatically turned for raising the supporting block after each cut by the hatch and pawl mechanism on the support (6).
U.S. Pat. No. 2,680,992 discloses an apparatus for slicing organic tissue wherein a blade (4) is placed in a reciprocating frame (5) so that a slice of tissue is made with each downward stroke. The tissue is placed in a cylindrical chamber (26) and pressed against a specimen support (3) after first being placed in a chamber and cooled. A slot (27) is formed between the chamber and specimen holder (3) through which the knife passes.
U.S. Pat. No. 3,420,130 shows a microtome with synchronized cutting and feeding operations. The motor revolves a screw shaft (53 and 71) which revolves to advance a carriage (21) carrying the specimen. Simultaneously, the cam shaft (39) rotates and with it the cam follower (86) carrying the knife lever (82). The knife lever carries the blade (103) and cyclically oscillates in an arc. The cam follower is held in close relationship by spring (87) engaging a pin (88), projecting from the knife lever, and by one adjusting screw (89). Whenever a cut is made on the block the carriage containing the specimen is stationary.
U.S. Pat. No. 3,440,913 teaches using a reciprocating knife for making cuts on a specimen wherein the knife edge moves transversely with respect to the specimen. Within casings (2 and 2') are a pair of leaf springs (4 and 4') attached by screws (5) to the inner wall of the casings. A shaft (3) is fixed on the springs and carries the blade holder (17). The shaft oscillates along its longitudinal axis by virtue of the transverse flexure of springs. The speed, frequency and amplitude of operation can be varied. The specimen is moved into the vibrating blade and after this operation the specimen carriage is returned and the knife stopped so that the section will float clear of the specimen.
U.S. Pat. No. 3,496,819 is another example of an oscillating knife for sectioning tissues mounted on an intermittently moving stage. The apparatus is similar to that of U.S. Pat. No. 3,420,130 above. The thickness of sections cut by knife (136) is controlled by a knob (216) (FIG. 2) which sets into play a series of motions which determines the transverse distance of the disc (226) upon which the specimen is resting.
U.S. Pat. No. 3,799,029 teaches reciprocating a knife along a slanted table for trimming a specimen prior to microtoming. A carriage assembly is mounted to a base and the reciprocating knife is attached to the assembly (19) to reciprocate therewith. The specimen is mounted in a vertical shaft (43) which can be turned to make different cuts by a goniometer. The carriage assembly is supported from two guide rails (53a and 53b) on two parallel arrays of bearings (57).
U.S. Pat. No. 3,975,977 teaches a crank (13) for downward movement of knife (7) and frame (79) and guides (11 and 12). The knife is set for automatic downward movement at a predetermined distance corresponding to the desired thickness to be taken from the object (3) upon movement of carriage (1) to the right.
Russian Patent No. 638,862 discloses the use of piezo-electric effects for fine adjustment of a specimen holder.
The publication "A Cantilever Microtome for Precision Sectioning in Electron and Light Microscopy", The Review of Scientific Instruments, Vol. 23, No. 11 (November 1952), pages 615-618, discloses a cantilever microtome wherein either the specimen or the knife may be mounted on the cantilever while the other is rigidly attached to the frame supporting the cantilever.
Despite advances in the art of microtomes, an existing problem associated with microtomes is that specimen blocks may often expand or contract unpredictably between cuts, especially in the case of thermal expansion due to thermal inputs to the specimen block. As a result of unpredictable expansion or contraction of the specimen, section thickness will not be accurate, and can in fact be unsuitable for desired examination procedures.
Accordingly, there exists a need for microtomes which produce accurate sectioning thickness regardless of unpredictable expansion or contraction of the specimen.